FIGS. 1 and 2 show an example of a prior art print medium support mechanism 20 and associated printhead 22. These example components are like those of the "DESKJET 560C" color ink-jet printer, manufactured by the Vancouver, Wash., Division of Hewlett-Packard Company. Print medium support mechanism 20 comprises, generally, a plurality of drive rollers or drive roller tires 24 distributed laterally across the width of a paper path. Drive rollers 24 are mounted on a roller shaft 26 which extends laterally across the paper path. For illustration purposes, a partial sheet of paper 28 is shown entering the print medium support mechanism beneath drive rollers 24. The paper is held against rollers 24 by pinch rollers, not shown, and by an upper print media guide 30. The paper wraps upward and around the rollers, and exits the support mechanism at the top of the rollers after making an approximately 180.degree. turn.
Printhead 22 comprises, generally, a carriage 34, a color cartridge 36, and a black cartridge 38. It has a number of nozzles (not shown) which are directed downwardly to deposit ink droplets on paper 28. The printhead is supported at a fixed elevation by a carriage rod (not shown). The printhead is designed to repeatedly traverse the lateral width of the underlying paper, while print medium support mechanism 20 feeds the paper longitudinally beneath the printhead.
The nozzles of printhead 22 are aligned longitudinally over the paper for a length along the paper path of approximately 1/6 of an inch. The area beneath the nozzles covered by a single lateral traverse of the printhead is referred to as a "print zone." To optimize print quality, it is desirable to keep the paper at a very specific distance from the nozzles as it travels through the print zone. If the paper is too far from the nozzles, accuracy and print quality will suffer. If the paper is too near to the nozzles, the paper may contact the nozzles and smear the applied ink. Paper positioning is complicated in ink-jet printers by the tendency of paper to buckle or bow when wet ink is applied.
Printers have print medium support mechanisms for correctly positioning paper relative to printheads. With economical manufacturing tolerances, however, it is sometimes difficult to maintain precisely the correct media-to-printhead spacing from printer to printer. Accordingly, print medium support mechanisms often have mechanical adjustments for establishing a desired printer-to-printhead spacing for a particular printer. These adjustments might be set during or immediately after manufacture, or by the ultimate user of the printer after purchase.
In print medium support mechanism 20 of FIGS. 1 and 2, paper positioning is accomplished by what is referred to as a pivot, generally referenced by the numeral 40. Pivot 40 is mounted to roller shaft 26 and pivots thereabout between retracted and non-retracted positions. Pivot 40 is shown in its non-retracted position in each of FIGS. 1 and 2. In this position, pivot 40 has a lower print media guide or guide surface 42 upon which paper 28 rests as it travels through the print zone beneath printhead 22. During printing, pivot 40 is in the non-retracted position to support paper 28 as shown. Pivot 40 retracts (in a clockwise direction as viewed in FIG. 2), thereby lowering guide surface 42, as a new sheet of paper is fed into the mechanism. Pivot 40 is also retracted as a printed sheet is ejected into an output tray. Components of a clutch 44 are shown in FIGS. 1 and 2. Clutch 44 performs the function of retracting pivot 40 at appropriate times under the control of operating logic which will not be discussed here.
Guide surface 42 extends laterally across the paper path, beneath a lower support edge of upper print media guide 30. The vertical position of this upper surface determines the vertical position of paper 28 and, therefore, the distance or spacing between paper 28 and the nozzles of printhead 22. In order to optimize this spacing and to account for manufacturing tolerances, an adjustable stop is provided in print medium support mechanism 20 for limiting the rotation of pivot 40 toward its non-retracted position and, consequently, the upward movement of guide surface 42. This stop in effect defines the non-retracted position of pivot 40 and of guide surface 42. The stop in the mechanism of FIGS. 1 and 2 comprises a finger 46 (FIG. 2) which extends inward from upper print media guide 40 and which hits a tab 50 on clutch 44 when the pivot is rotated toward its non-retracted position. Since pivot 40 rotates with clutch 44, tab 50 limits the rotation of pivot 40 and therefore establishes the orientation of pivot 40 and the vertical elevation of guide surface 42 when the pivot is not retracted. The finger can be manually adjusted to vary the point at which it hits tab 50, thereby establishing the non-retracted elevation of guide surface 42. This adjustment is normally performed during manufacture of a printer to attain a desired paper elevation while also maintaining a specified minimum gap between guide surface 42 and the upper print media guide 30. The specified minimum gap is just large enough to allow passage of the thickest contemplated print medium, such as perhaps an envelope.
The support mechanism described above is designed to support paper through a print zone of about 1/6 of an inch. Newer printer designs, however, seek to incorporate printheads with longer print zones such as 1/2 an inch or more. Longer print zones are desirable to increase printing speeds. However, it is much more difficult to maintain precise paper positioning along a print zone of this length. Adding to this difficulty is the tendency of paper to bulge or curve upwardly (toward the printhead) in areas where ink is applied. This tendency is much more dramatic with long print zones, in which the potential area of ink application is much larger. Any significant bulge, however, will cause the paper to hit the printhead nozzles, thus smearing the ink. Moving the paper away from the printhead to avoid this interference decreases nozzle accuracy and reduces the overall print quality. It would be desirable to control the paper in such a way as to eliminate or at least drastically reduce its tendency to buckle or bulge. This degree of control has not been attained in the past.